This invention relates to a method for manufacture of shotcrete structures utilizing shorcreting practices to provide structures having high impact resistance and optimum deformation properties (slight shrinkage and creep tendencies, reduced crack formation and improved crack dispersion) by means of shotcreting, in which a dry mixture of a binding agent and a ballast material is carried in an air stream through a first tubing to a spraying nozzle in which water is added to said mixture.
The term shotcreting means the introduction of a dry, pneumatically conveyed mixture of a binding agent and a ballast material to nozzle, where water is added. The water-cement ratio is much lower than for ordinary concrete, about 0.3-0.4, which means that the mixture leaving the nozzle is relatively dry. This shotcrete mixture leaves the nozzle at a very high velocity. The ratio dry material--air is as low as about 0.01.
Attempts have earlier been made to reinforce shotcrete materials with fibres. The orientation and location of the fibres in the concrete is an essential factor. A number of different mixting methods have been developed, but none of these has proved to be acceptable in practice, due to difficulties in distribution of said fibres. The fibres cluster into large units, which will be located in the bottom layer of the structure due to their heaviness. The mixing of fiber shotcrete is made difficult thereby that the result is heavily dependent on the type of fibrous material used the quantity of the fibrous material and the length and thickness respectively of the individual fibres.
Attempts carried out with admixture of fibres in concrete have shown that considerably improved properties can be achieved for the concrete material, if the alignment and the distribution of the fibres can be controlled.
Earlier has been proposed (U.S. Pat. No. 3,289,371) a method of applying a gypsum slurry, to which is added to reinforcing material, onto a surface. This slurry is pumped through the nozzle at a low velocity, as compared to shotcreting where the mixture is pneumatically conveyed at a high speed. The air content is 0 (compared to a shotcrete mixture, where the air content is about 0.99), and reinforcing glass fibers are introduced to the slurry either after the outlet of the nozzle or immediately before the outlet. The introduction of the reinforcing fibers downstream of the outlet of the nozzle will not afford sufficient mixing of fibers with a slurry, but the reinforcement is preferably concentrated in the upper stratum of the gypsum.
The introduction of reinforcing fibers immediately before the outlet of the nozzle would not function at all, since the slurry would probably plug up the outlet of the nozzle, because the smaller cross section area available of the axially introduced tube or nozzle.
If an introduction of reinforcement fibers were effected immediately before the outlet of a shotcrete nozzle, the fibers tend to concentrate in the middle of the stream leaving the nozzle. Some mixing of the fibers with the shotcrete mixture would of course take place in the areas between the inner fiber stream and the outer shotcrete mixture stream, but there would be no homogeneous mixing in the entire stream.